Hypothesis (H) theory characterizes human problem solving (limited, so far, to the discrimination-learning and concept-learning situations) as follows: the S learns by testing Hs according to a more-or-less efficient strategy. Probes for detecting S's H are well developd; techniques for detecting his strategy have just been formulated. The general objectives of the proposed research are three-fold: (1) to document the details of the processes underlying the H-testing process. Thus, perceptual and memory processes will be studied to determine how Ss "select a sample of Hs", "recode the H set after a wrong", etc. (2) to improve the technique for detecting Ss strategy and to study the strategies of Ss under a variety of conditions. The strategies-analysis will be applied to children of different ages, in an attempt to describe the shift with age from primitive to more sophisticated strategies. Also, methods (instructions, motivational variations) for improving Ss strategy will be investigated. (3) to expand the theory both by incorporating processes other than H testing which contribute to problem solving and by applying the theory to new problem-solving situations.